Most present processes for zinc production require oxidic raw materials. Nowadays, the most widely used process for zinc production is the sulphate process which represented 80% of total world production in 1997. It consists of the steps of roasting, leaching, liquor purification, electro-deposition, melting and casting. The most abundant natural raw material is zinc blende (ZnS), which must be roasted to ZnO as a first step. Of course, zinc oxide recycled from other industries can be used directly without roasting.
The zinc electro-deposition is readily disturbed by extremely low levels of impurities in the electrolyte; the electrolyte liquor must therefore be rigorously purified. This means that the concentration of all the elements other than zinc present in the feed has to be lowered drastically. For instance, the following limits are generally applied: Fe less than 5 ppm, Cu less than 0.1 ppm, Co less than 0.2 ppm, As less than 0.01 ppm, Ge less than 0.005 ppm etc. In consequence, the liquor purification step is rather complicated, involves a lot of different process steps and produces many different types of residues. Some of these residues necessitate further treatment to valorise the metals they contain. Other residues, among which large quantities of iron residue in the form of jarosite or goethite, must be disposed of in ponds, a practice which is under strong environmental pressure.
Another difficulty of the sulphate process is that in order to achieve maximum recovery yields of zinc and also of other valuable impurities such as copper and cadmium, the leaching step is long and complicated, involving successive leaching operations at different acidities and temperatures.
At the turn of the 20th century, Swinburne and Ashcroft proposed a completely different process for the production of zinc from sulfidic material in GB 14,278. This process is known as ‘chlorine smelting with fused chloride electrolysis’. The method consists in:    converting all the metals present in a sulphide ore or concentrate feed into chlorides by reacting them with chlorine, and producing elemental sulphur at the same time;    eliminating all the chlorides other than ZnCl2 by cementation with zinc metal; and    electrolysing the fused zinc chloride to produce pure zinc and to recover the chlorine.
Basically, in a first embodiment of the Ashcroft process called the ‘wet’ embodiment, the chlorination reaction produces a chloride melt, containing insoluble oxides and gangues which were not readily converted to chlorides during the chlorination step. The whole melt is then dissolved in water, in order to separate the insoluble fraction, and subjected to purification. Iron and manganese are eliminated by the addition of zinc oxide and chlorine in the aqueous solution. The other metals are cemented by the addition of Pb and/or Zn metal. The purified zinc chloride solution is then boiled down again, and a preliminary electrolysis with a rough, consumable carbon anode effects the final de-hydration of the zinc chloride before the actual zinc electrolysis.
A variation of the process consists in the cementation of all the metals with zinc, without prior dissolution in water, and is called the ‘dry’ embodiment. The zinc chloride is also subjected to a pre-electrolysis to ensure a sufficiently low oxide and water content. Then the actual electrolysis is carried out as above to produce molten zinc.
The Ashcroft process, although very interesting, suffers from several drawbacks. First, a lot of zinc metal is consumed in the cementation step, whether in the dry or the wet embodiment. Also, as the molten zinc chloride must be very pure for the electrolysis, the totality of the other elements must be removed from the flow, which implies a non-selective output which is difficult to valorise. Finally, particularly in the wet embodiment, a lot of energy is required to evaporate the water, and a pre-electrolysis with consumable anodes is needed to completely dry the melt. These operations are not economically interesting.